U.S. patent application number 16/612641 was filed with the patent office on 2020-03-26 for power conversion device, power conversion system, and power conversion device operation method.
This patent application is currently assigned to Mitsubishi Electric Corporation. The applicant listed for this patent is Mitsubishi Electric Corporation. Invention is credited to Takahiro KATO, Yu KAWAI.
Application Number | 20200099305 16/612641 |
Document ID | / |
Family ID | 65002282 |
Filed Date | 2020-03-26 |
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United States Patent
Application |
20200099305 |
Kind Code |
A1 |
KAWAI; Yu ; et al. |
March 26, 2020 |
POWER CONVERSION DEVICE, POWER CONVERSION SYSTEM, AND POWER
CONVERSION DEVICE OPERATION METHOD
Abstract
A power conversion device is configured with a plurality of
power conversion units multiplexed in parallel each having a power
converter for converting a power between a power source and a
common load. At least one of the power conversion units is provided
with an operation manager for managing the operation of the power
converter. The operation manager changes the proportional gain of a
voltage adjuster that performs a proportional control by inputting
the steady-state offset between a target voltage and a voltage to
the load, to determine, by comparing a change in the steady-state
offset with a change in the proportional gain, whether the at least
one power conversion unit is in a single operation in which the
other power conversion units are in no operation.
Inventors: |
KAWAI; Yu; (Chiyoda-ku,
JP) ; KATO; Takahiro; (Chiyoda-ku, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation |
Chiyoda-ku |
|
JP |
|
|
Assignee: |
Mitsubishi Electric
Corporation
Chiyoda-ku
JP
|
Family ID: |
65002282 |
Appl. No.: |
16/612641 |
Filed: |
February 16, 2018 |
PCT Filed: |
February 16, 2018 |
PCT NO: |
PCT/JP2018/005405 |
371 Date: |
November 11, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 1/102 20130101;
H02M 3/156 20130101; H02M 3/158 20130101; H02M 7/219 20130101; H02M
7/08 20130101; H02M 3/1584 20130101; H02J 7/0063 20130101; H02J
2207/20 20200101 |
International
Class: |
H02M 3/158 20060101
H02M003/158 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 2017 |
JP |
2017-136991 |
Claims
1: A power conversion device comprising: a plurality of power
conversion units configured in parallel and connected a load, each
power conversion unit having: a power converter configured to
convert a power from a power source to a DC power for the load; and
a voltage controller provided with a voltage adjuster configured to
receive a steady-state offset between a target voltage and a
voltage output to the load from the power converter, to perform a
proportional control for controlling the voltage to the target
voltage, wherein at least one of the plurality of power conversion
units has an operation manager configured to manage an operation of
the at least one power conversion unit, and the operation manager
changes a proportional gain for the proportional control of the
voltage adjuster of the at least one power conversion unit to
determine whether or not the at least one power conversion unit is
in a single operation in which other of the power conversion units
except for the at least one power conversion unit are not in
operation, by comparing a change in the steady-state offset with a
change in the proportional gain.
2: The power conversion device of claim 1, wherein the change in
the steady-state offset is compared with the change in the
proportional gain by calculating, using the voltage to the load and
a target current value output from the voltage adjuster, estimated
resistance values or estimated power values of the load before and
after the proportional gain is changed.
3: The power conversion device of claim 1, wherein at least two of
the power conversion units each have the operation manager, and
individual identification numbers are respectively assigned to the
at least two power conversion units to differentiate, for each
identification number, timings of starting a process of changing
the proportional gain to determine whether or not one of the at
least two power conversion units is in the single operation.
4: The power conversion device of claim 1, wherein the operation
manager of one of the power conversion units pauses a power
conversion operation of the one power conversion unit when
determines the one power conversion unit to be not in the single
operation but in a parallel operation with the other of the power
conversion units when a power from the one power conversion unit is
equal to or smaller than a preset power threshold for the one power
conversion unit.
5: The power conversion device of claim 4, wherein the operation
manager of one of the power conversion units starts the power
conversion operation of the one power conversion unit when the
voltage to the load becomes equal to or lower than a preset
reference voltage lower than the target voltage after the power
conversion operation of the one power conversion unit is
paused.
6: The power conversion device of claim 4, wherein the operation
manager of one of the power conversion units starts the power
conversion operation of the one power conversion unit when the
voltage to the load becomes equal to or higher than a preset
reference voltage higher than the target voltage after the power
conversion operation of the one power conversion unit is
paused.
7: A power conversion system comprising: the power conversion
device recited in claim 6 and a load including a power generation
mechanism connected to the power conversion device.
8: A method of operating a power conversion device that has a
plurality of power conversion units configured in parallel and
connected to a load, and each power conversion unit includes a
power converter connected between a power source and the load and
is configured to convert a power from the power source to a DC
power for the load; and a voltage controller provided with a
voltage adjuster configured to receive a steady-state offset
between a target voltage and a voltage output to the load from the
power converter, to perform a proportional control for controlling
the voltage to the load to the target voltage, the method of
operating the power conversion device comprising steps of:
determining whether or not other of the plurality of power
conversion units except for at least one of the power conversion
units are in a parallel operation with the at least one power
conversion unit, by changing a proportional gain of the
proportional control of the voltage adjuster of the at least one
power conversion unit and by comparing a change in the steady-state
offset with the change in the proportional gain; and pausing a
power conversion operation of the at least one power conversion
unit when the power supplied to the load from the at least one
power conversion unit is equal to or smaller than a preset power
threshold for the at least one power conversion unit and when the
other power conversion units are determined to be in the parallel
operation with the at least one power conversion unit.
9: The method of operating the power conversion device, of claim 8
further comprising: a step of starting the power conversion
operation of the at least one power conversion unit when the
voltage to the load becomes equal to or lower than a preset
reference voltage lower than the target voltage after the power
conversion operation of the at least one power conversion unit is
paused.
10: The method of operating the power conversion device, of claim 8
further comprising: a step of starting the power conversion
operation of the at least one power conversion unit when the
voltage to the load becomes equal to or higher than a preset
reference voltage higher than the target voltage after the power
conversion operation of the at least one power conversion unit is
paused.
11: The power conversion device of claim 2, wherein at least two of
the power conversion units each have the operation manager, and
individual identification numbers are respectively assigned to the
at least two power conversion units to differentiate, for each
identification number, timings of starting a process of changing
the proportional gain to determine whether or not one of the at
least two power conversion units is in the single operation.
12: The power conversion device of claim 2, wherein the operation
manager of one of the power conversion units pauses a power
conversion operation of the one power conversion unit when
determines the one power conversion unit to be not in the single
operation but in a parallel operation with the other of the power
conversion units when a power from the one power conversion unit is
equal to or smaller than a preset power threshold for the one power
conversion unit.
13: The power conversion device of claim 3, wherein the operation
manager of one of the power conversion units pauses a power
conversion operation of the one power conversion unit when
determines the one power conversion unit to be not in the single
operation but in a parallel operation with the other of the power
conversion units when a power from the one power conversion unit is
equal to or smaller than a preset power threshold for the one power
conversion unit.
14: The power conversion device of claim 11, wherein the operation
manager of one of the power conversion units pauses a power
conversion operation of the one power conversion unit when
determines the one power conversion unit to be not in the single
operation but in a parallel operation with the other of the power
conversion units when a power from the one power conversion unit is
equal to or smaller than a preset power threshold for the one power
conversion unit.
Description
TECHNICAL FIELD
[0001] The present application relates to a power conversion device
that is configured with power conversion units multiplexed in
parallel.
BACKGROUND ARTS
[0002] The parallel multiplex configuration of power conversion
units receives attention as a design facilitating technology that
eliminates the need of a custom design meeting required
specifications. In a case of controlling the voltage at the
parallel connected ports of a parallel system in which the control
functions of power conversion units are independent from each
other, a power averaging function is necessary to avoid power
concentration to a specific one of the power conversion units (see,
for example, Patent Document 1).
PRIOR ART DOCUMENT
Patent Document
[0003] Patent Document 1: JP2010-11567 A
SUMMARY OF THE INVENTION
Problem that the Invention is to Solve
[0004] Since the switching power supplies disclosed in Patent
Document 1, which correspond to the power conversion units, have a
drooping characteristic, it is possible to mitigate a problem of
concentrating the power to a specific one of the units when they
are multiplexed in parallel. However, since it is impossible to
detect whether or not one of the units is in a single operation in
the parallel multiplexed configuration, stopping some of the units
to improve the efficiency is difficult in light of continuous
operation, thus requiring for all power conversion units to always
output the power.
[0005] The present application is made to resolve the
above-described problem, and aimed at providing a power conversion
device that is configured with parallel multiplexable power
conversion units in which one of the power conversion units is able
to determine whether or not it is in a single operation without
using information from outside the one power conversion unit.
Means for Solving the Problem
[0006] A power conversion device disclosed in the present
application includes a plurality of power conversion units
configured in parallel and connected a load, each power conversion
unit having: a power converter configured to convert a power from a
power source to a DC power for the load; and a voltage controller
provided with a voltage adjuster configured to receive a
steady-state offset between a target voltage and a voltage output
to the load from the power converter, to perform a proportional
control for controlling the voltage to the target voltage, wherein
at least one of the plurality of power conversion units has an
operation manager configured to manage an operation of the at least
one power conversion unit, and the operation manager changes a
proportional gain for the proportional control of the voltage
adjuster of the at least one power conversion unit to determine
whether or not the at least one power conversion unit is in a
single operation in which other of the power conversion units
except for the at least one power conversion unit are not in
operation, by comparing a change in the steady-state offset with a
change in the proportional gain.
Advantage Effect of the Invention
[0007] A power conversion device disclosed in the present
application is configured with a plurality of power conversion
units having output ports connected in parallel, and one of the
power conversion units is able to determine alone whether or not
the one power conversion unit is in the single operation without
using information from outside the one power conversion unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram showing a configuration of a power
conversion system including a power conversion device according to
Embodiment 1;
[0009] FIG. 2 is a block diagram showing a configuration of the
power conversion system including another power conversion device
according to Embodiment 1;
[0010] FIG. 3 is a circuit diagram showing an example of a detailed
configuration of the power conversion unit shown in FIG. 1;
[0011] FIG. 4 is a circuit diagram showing an example of a detailed
configuration of the power conversion unit shown in FIG. 2;
[0012] FIG. 5 is a control block diagram schematically showing a
control system of the power conversion unit of the power conversion
device according to Embodiment 1;
[0013] FIG. 6 is a schematic diagram for explaining an operation of
the power conversion device according to Embodiment 1 when one of
the power conversion units is in the single operation;
[0014] FIG. 7 is a schematic diagram for explaining an operation of
the power conversion device according to Embodiment 1 when some of
the power conversion units are in the parallel operation;
[0015] FIG. 8 is a schematic diagram for explaining another
operation of the power conversion device according to Embodiment 1
when some of the power conversion units are in the parallel
operation;
[0016] FIG. 9 is a flowchart for explaining the operation of the
power conversion device according to Embodiment 1;
[0017] FIG. 10 is a flowchart for explaining an operation of a
power conversion device according to Embodiment 2;
[0018] FIG. 11 is a graph showing a specific example of a
characteristic of the power conversion device according to
Embodiment 2;
[0019] FIG. 12 shows graphs for explaining the effect of the power
conversion device according to Embodiment 2;
[0020] FIG. 13 is a block diagram showing a configuration of a
power conversion system including the power conversion device,
according to Embodiment 3; and
[0021] FIG. 14 is a block diagram showing an example of a hardware
configuration of the voltage controller and the operation manager
of the power conversion device.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
Embodiment 1
[0022] FIG. 1 is a block diagram showing a configuration of a power
conversion system including a power conversion device according to
Embodiment 1. The power conversion device is configured with N
power conversion units 10-1, 10-2, . . . , 10-N (occasionally,
individual power conversion units are representatively referred to
as "power conversion units 10"), and each power conversion unit has
a power converter 11 to supply power to a common load 30. The power
conversion units 10-1, 10-2, . . . , 10-N shown in FIG. 1 are
configured to receive DC from DC power sources 20-1, 20-2, . . . ,
20-N, respectively, and to output a DC voltage (DC bus voltage)
Vout. In other words, the power converter 11 of each power
conversion unit is a DC-DC converter.
[0023] FIG. 2 is a block diagram showing a configuration of the
power conversion system including another power conversion device
according to Embodiment 1. The power conversion device is
configured with N power conversion units 10-1, 10-2, . . . , 10-N
(occasionally, the individual power conversion units are
representatively referred to as "power conversion units 10"), and
each power conversion unit has a power converter 11 to supply power
to the common load 30. The respective power conversion units 10-1,
10-2, . . . , 10-N shown in FIG. 2 are configured to receive AC
from AC power sources 21-1, 21-2, . . . , 21-N and to output the DC
voltage (DC bus voltage) Vout. In other words, the power converter
11 of each power conversion unit is an AC-DC converter.
[0024] The power conversion units 10 in FIG. 1 and also the power
conversion units 10 in FIG. 2 each are provided with a voltage
controller 12 for controlling to a target voltage Vout* the DC bus
voltage Vout, which is the voltage to the load, detected by a
voltage sensor 15 and an operation manager 13 for changing a set
value for the voltage controller 12 using the steady-state offset
(Vout*-Vout) between the target voltage Vout*and the DC bus voltage
Vout. The operation manager 13 has a function of determining
whether or not the host power conversion unit is in a single
operation on the basis of the set value for the voltage controller
12 and of the steady-state offset.
[0025] To be more specific, the voltage controller 12 and the
operation manager 13 are each made up of a computer processor 101
such as a central processing unit (CPU), a storage memory 102 for
exchanging data with the computer processor 101, an I/O interface
103 for inputting/outputting signals between the computer processor
101 and the external, and the other components, as shown in FIG.
14. The computer processor 101 may include an application specific
integrated circuit (ASIC), an integrated circuit (IC), a digital
signal processor (DSP), a field programmable gate array (FPGA), and
various types of signal processing circuits. In addition, the
voltage controller 12 and the operation manager 13 may be made up
of one computer processor 101. In further addition, the computer
processor 101 may be replaced with a plurality of processors of the
same type or a plurality of processors of different types to share
processing of the voltage controller 12 and the operation manager
13. The storage memory 102 has memory devices such as a random
access memory (RAM) configured for the computer processor 101 to be
able to read data therefrom and write data thereinto and a read
only memory (ROM) configured for the computer processor 101 to be
able to read data therefrom. The I/O interface 103 is made up of,
for example, an A/D converter for inputting to the computer
processor 101 the signals output from the voltage sensor 15, a
drive circuit for outputting signals to the power converter 11, and
the like.
[0026] The configuration and operation of the power conversion
units according to Embodiment 1 are described below. FIGS. 3 and 4
are block diagrams respectively showing a specific example of the
power converter 11 and the voltage controller 12 shown in FIG. 1
and a specific example of the power converter 11 and the voltage
controller 12 shown in FIG. 2.
[0027] As shown above, the power converter of each power conversion
unit 10 may be the DC-DC converter as shown in FIGS. 1 and 3 or the
AC-DC converter as shown in FIGS. 2 and 4, and may further have a
mixed configuration such that the power converter 11 of a certain
power conversion unit is the DC-DC converter and the power
converter 11 of another power conversion unit is the AC-DC
converter. The power sources in the input sides of the respective
power conversion units may have any configuration, such as a common
source or separated sources, or sources common to part of the power
conversion units. Moreover, the number N of parallel power
conversion units may be any value. Furthermore, the load is not
limited to a simple resistive load.
[0028] FIG. 5 is a control block diagram schematically showing a
control system of each power conversion unit having the
configuration shown in FIGS. 3 and 4. The gain of "ACR" shown in
FIG. 5, which means simplification of a control system including a
current control system, is treated as one in the following
description.
[0029] The current control system shown in FIGS. 3 and 4 included
in a configuration expressed simply as the ARC shown in FIG. 5 is
configured to receive a target value Io1* of the DC bus current
from a voltage adjuster 121 to control the duty D1 of the power
conversion unit so that a target input current value IL1* obtained
by converting the target value Io1* of the DC bus current using the
ratio between a voltage Vin1 input to the power conversion unit and
the DC bus voltage Vout agrees with a detected current value
IL1.
[0030] Note that the power conversion device disclosed in the
present application is feasible for a case where the voltage
controller 12 for controlling the DC bus voltage Vout to the target
voltage Vout* give a drooping characteristic to the DC bus voltage
in response to the output power, and the configuration of the
voltage adjuster 121 in the voltage controller 12 is not limited to
the configuration for the proportional control (P-control) shown in
FIGS. 3 and 4. The voltage adjuster 121 only needs to have a
control system that inputs the steady-state offset between the
target voltage Vout* and the actual voltage Vout, and performs a
proportional control to approximate the steady-state offset to zero
by multiplying a value based on the steady-state offset by the
proportional gain, irrespective of whether the value has a
linearity such as the steady-state offset itself or a non-linearity
such as the square or the square root of the steady-state
offset.
[0031] An operation of the operation manager 13 of Embodiment 1 is
described. The description uses the simplified control block
diagram shown in FIG. 5. In addition, the voltage adjuster 121
shown in FIG. 5 can be replaced with a control mechanism having a
characteristic of changing the steady-state offset (Vout*-Vout)
between the target voltage Vout* and DC bus voltage Vout in
response to the input power or the output power of the power
conversion unit. In further addition, the configuration subsequent
to the voltage adjuster 121 is not limited to that shown in FIG.
5.
[0032] The operation of the power conversion device disclosed in
the present application is based on the an idea that when the gain
of the voltage adjuster 121 of the voltage controller 12 of a
certain one of the power conversion units is changed, change of the
steady-state offset (Vout*-Vout) between the target voltage Vout*
and the DC bus voltage Vout will be different between when the one
power conversion unit is in the single operation and when the one
power conversion unit and the other power conversion units
connected in parallel therewith operate in parallel. In the single
operation, when the gain is changed, the steady-state offset is
changed by an amount corresponding to the gain change. In the
parallel operation in which the plurality of power conversion units
supply power to the load, however, when the gain of a certain one
of the power conversion units changes, the steady-state offset
(Vout*-Vout) changes, in association with the gain change, by only
less than the change amount corresponding to the gain change
because the other power conversion units complement the power. For
example, in performing the proportional control for the
steady-state offset (Vout*-Vout), when the gain is reduced to half,
the steady-state offset increases about two times in the single
operation but the steady-state offset change in the parallel
operation is smaller than that in the single operation. Thus, by
comparing the change in the steady-state offset associated with the
gain change with the change in the gain of a certain one of the
power conversion units, it is possible to determine whether the one
power conversion unit is in the single operation or in the parallel
operation with the other power conversion units. However, since the
steady-state offset has a very small value, it is conceivable that
the determination can be made incorrectly due to a large error even
if the value is directly observed. In the following, a method is
proposed that facilitates observation of change of the steady-state
offset by calculating stable parameters that correspond to the
steady-state offset.
[0033] The method is described below, taking the configuration of
FIG. 5 as an example, that the voltage adjuster 121 performs
proportional control for the steady-state offset (Vout*-Vout)
itself. First, a description is made of the relationship between a
DC bus current Ioi output from one of the power conversion units in
association with change of the proportional gain of the voltage
adjuster 121 and the steady-state offset between the target voltage
Vout* of the DC bus and the actual voltage Vout thereof. In FIG. 5,
the relationship between the target voltage Vout* of the DC bus and
the actual voltage Vout thereof can be expressed by Eq. (1):
Vout Vout * = K i = 1 N Coi s + K i = 1 N Coi = .omega. K s +
.omega. K , ( 1 ) ##EQU00001##
where K is the proportional gain; Co is a capacity of the output
capacitance provided to the output side of each power conversion
unit; and Coi denotes the capacity of the output capacitance of the
i-th power conversion unit 10-i, and the constant .omega..sub.K is
substituted for the terms of the 0-th order of the function s in
the denominator and the numerator on the right hand side because it
is common to the denominator and the numerator.
[0034] Here, attention is paid to the first power conversion unit
10-1. The relationship between the actual voltage Vout and a
current disturbance, which is the difference between the summation
of output currents (Io2+Io3+ . . . +IoN) of the other power
conversion units and a load current Iload, can be expressed by Eq.
(2):
Vout Io 2 + Io 3 + + IoN - Iload = 1 i = 1 N Coi s + K i = 1 N Coi
= 1 K .omega. K s + .omega. K . ( 2 ) ##EQU00002##
[0035] Referring to FIG. 5, the relationship between the target DC
bus voltage Vout* calculated from Eqs. (1), (2) and the actual
voltage Vout calculated from the current disturbance, which is the
difference between the summation of the output currents of the
other power conversion units and the load current, is expressed as
Eq. (3):
Vout = .omega. K s + .omega. K { Vout * + 1 K ( Io 2 + Io 3 + + IoN
- Iload ) } . ( 3 ) ##EQU00003##
[0036] Here, an influence of the voltage controllers of the other
power conversion units is considered. When all power conversion
units have a common proportional gain, Eqs. (1), (2), (3) can be
replaced with Eqs. (4), (5), (6), respectively:
Vout Vout * = NK i = 1 N Coi s + NK i = 1 N Coi = .omega. K 0 s +
.omega. K 0 , ( 4 ) Vout - Iload = 1 i = 1 N Coi s + NK i = 1 N Coi
= 1 NK .omega. K 0 s + .omega. K 0 , and ( 5 ) Vout = .omega. K 0 s
+ .omega. k 0 ( Vout * - 1 NK Iload ) , ( 6 ) ##EQU00004##
where the constant .omega..sub.K0 is substituted for terms of the
0-th order of the function s in the denominator and the numerator
on the right hand sides because it is common to the denominator and
the numerator.
[0037] Since the value of .omega..sub.K0/(s+.omega..sub.K0) is one
in the steady state, in a situation of the load being considered as
a constant resistive load R, the relationship obtained from Eq. (6)
is expressed as Eq. (7):
R = Vout Iload = Vout * Iload - 1 NK . ( 7 ) ##EQU00005##
[0038] Furthermore, in a situation of the load being considered as
a constant power load, the relationship obtained from Eq. (6) is
expressed for the power Pas Eq. (8):
P = Vout Iload = Vout * Iload - 1 NK Iload 2 . ( 8 )
##EQU00006##
[0039] Note that a load to which the power conversion device
disclosed in the present application is applicable only needs to be
a load that can be simulated as a constant resistive load or a
constant power load by being time averaged even though it is a
non-linear load causing an instantaneous power change. Thus, the
load is not limited to a constant resistive load pr a constant
power load with no time variation. Furthermore, the load may have a
power generation function, as described in Embodiment 3. The
following description is made by assuming that the load can be
simulated as a constant resistive load or a constant power load by
being time averaged. Hence, the load is referred to as a constant
resistive load or a constant power load in the description.
[0040] Each power conversion unit is able to estimate the load
using the detected DC bus voltage Vout of the target DC-bus current
value Io1* generated by the voltage adjuster 121 in the voltage
controller 12 of the host power conversion unit. To be more
specific, by considering that the target DC-bus current value Io1*
generated by the voltage adjuster 121 will be the current from the
power conversion unit to the load, the resistance value can be
estimated for when the load can be simulated as a constant
resistive load or the power can be estimated for when the load can
be simulated as a constant power load. Eq. (9) is an estimating
equation for calculating the estimated resistance value Rx in the
case of the constant resistive load, and Eq. (10) is an estimating
equation for calculating the estimated power value Px in the case
of the constant power load.
Rx = Vout Io 1 * = Vout K ( Vout * - Vout ) and ( 9 ) Px = Vout Io
1 * = Vout K ( Vout * - Vout ) . ( 10 ) ##EQU00007##
[0041] In the following, a description is made of a principle of a
method of determining the operation to be single using the actual
relationship of the load characteristics shown by Eqs. (7), (8) and
the estimating equations shown by Eqs. (9), (10). While the
description is made using configurations with one power conversion
unit (single operation) and with three power conversion units, no
limitation is imposed to the number of parallel power conversion
units.
[0042] FIG. 6 is a diagram schematically showing the single
operation of the one power conversion unit and FIG. 7 is a diagram
schematically showing the parallel operation of the three power
conversion units. In the single operation, a relationship
corresponding to Eq. (6) is expressed as Eq. (11):
Vout = .omega. K 0 s + .omega. K 0 ( Vout * - 1 K Iload ) . ( 11 )
##EQU00008##
Further, in the three-unit parallel operation, a relationship
corresponding to Eq. (6) is expressed as Eq. (12):
Vout = .omega. K 0 s + .omega. K 0 ( Vout * - 1 3 K Iload ) . ( 12
) ##EQU00009##
[0043] When the proportional gain of the voltage adjuster 121 of
the one power conversion unit is changed and reduced from K to
(K-.DELTA.K), Eqs. (11), (12) can be expressed as Eqs. (13), (14),
respectively:
Vout = .omega. K 0 s + .omega. K 0 ( Vout * - 1 K - .DELTA. K Iload
) and ( 13 ) Vout = .omega. K 0 s + .omega. K 0 ( Vout * - 1 3 K -
.DELTA. K Iload ) . ( 14 ) ##EQU00010##
[0044] For the case of the constant resistive load in the steady
state, resistance estimating equations corresponding to Eqs. (11),
(12) for the single operation are expressed as Eqs. (15), (16),
respectively:
Rx = Vout K ( Vout * - Vout ) = Vout Iload and ( 15 ) Rx = Vout ( K
- .DELTA. K ) ( Vout * - Vout ) = Vout Iload . ( 16 )
##EQU00011##
[0045] Further, power estimating equations for the constant power
load are expressed as Eqs. (17), (18), respectively:
Px=VoutK(Vout*-Vout)=VoutIload (17) and
Px=Vout(K-.DELTA.K)(Vout*-Vout)=Vout-Iload (18)
Thus, it is found that in the single operation, when the load can
be simulated as a constant resistive load, the load takes the same
estimated resistance value before and after the proportional gain
is changed by .DELTA.K; or when the load can be simulated as a
constant power load, the load takes the same estimated power value
before and after the proportional gain is changed by .DELTA.K In
the single operation, with the change ratio (K-.DELTA.K)/K of the
proportional gain, the quotient (Vout*-Vout)/Vout of the
steady-state offset and the DC bus voltage for the constant
resistive load, and the product (Vout*-Vout)Vout of the
steady-state offset and the DC bus voltage for the constant power
load, are changed K/(K-.DELTA.K) times, which is the inverse of the
proportional gain change ratio. Consequently, the load estimation
calculated from Eqs. (9), (10) have the same value before and after
the proportional gain is changed.
[0046] In contrast to this, constant resistive load estimating
equations corresponding to Eqs. (12), (14) for the three-unit
parallel operation are expressed as Eqs. (19), (20):
Rx = Vout K ( Vout * - Vout ) = 3 Vout Iload and ( 19 ) Rx = Vout (
K - .DELTA. K ) ( Vout * - Vout ) = 3 K - .DELTA. K K - .DELTA. K
Vout Iload . ( 20 ) ##EQU00012##
[0047] Further, constant power load estimating equations are
expressed as Eqs. (21), (22):
Px = Vout K ( Vout * - Vout ) = Vout Iload 3 and ( 21 ) Px = Vout (
K - .DELTA. K ) ( Vout * - Vout ) = Vout K - .DELTA. K 3 K -
.DELTA. K Iload . ( 22 ) ##EQU00013##
[0048] Thus, it is found that in the three-unit parallel operation,
the constant resistive load or the constant power load takes
different estimated values before and after the proportional gain
is changed by .DELTA.K As described above, with the change ratio
(K-.DELTA.K)/K of the proportional gain in the single operation,
the quotient (Vout*-Vout)/Vout of the steady-state offset and the
DC bus voltage for the constant resistive load, and the product
(Vout*-Vout)Vout of the steady-state offset and the DC bus voltage
for the constant power load, are changed K/(K-.DELTA.K) times,
which is the inverse of the change ratio of the proportional gain.
Consequently, the load estimation calculated from Eqs. (9), (10)
have the same value before and after the proportional gain is
changed. In the parallel operation, on the other hand, with the
change ratio (K-.DELTA.K)/K of the proportional gain, the quotient
(Vout*-Vout)/Vout of the steady-state offset and the DC bus voltage
for the constant resistive load, and the product (Vout*-Vout)-Vout
of the steady-state offset and the DC bus voltage for the constant
power load, are changed differently from K/(K-.DELTA.K) times,
which is the inverse of the change ratio of the proportional gain.
Consequently, the load estimation calculated from Eqs. (9), (10)
takes different values before and after the proportional gain is
changed. For example, when .DELTA.K is set at half of the original
proportional gain, the estimated value calculated from Eq. (19) is
3/5 times smaller than that calculated from Eq. (20) and the
estimated value calculated from Eq. (21) is 5/3 times larger than
that calculated from Eq. (22). In this way, whether the operation
is single or parallel can be determined using the load estimating
equation (9) or (10). To be more specific, whether the operation is
single or parallel can be determined by calculating the estimated
resistance value Rx from Eq. (9) for the constant resistive load or
the estimated power value Px from Eq. (10) for the constant power
load, using the DC bus voltage Vout of the one power conversion
unit and Io1* output from the voltage adjuster 121.
[0049] The above showed that it is possible to determine whether
the operation is single or parallel by comparing the change in the
estimated value of the load with the change in the proportional
gain. As described above, comparing the change in the estimated
resistance value for the constant resistive load with the change in
the proportional gain is equivalent to comparison between the
change in (Vout*-Vout)/Vout and the change in the proportional
gain. Similarly, comparing the change in the estimated power value
for the constant power load with the change in the proportional
gain is equivalent to comparison between the change in
(Vout*-Vout)/Vout and the change in the proportional gain. The
ratio of the change in Vout here is small compared to the ratio of
the change in the steady-state offset (Vout*-Vout) in association
with the change in the proportional gain. Thus, comparing the
change in the estimated value of the load with the change in the
proportional gain substitutes for comparison between the change in
the steady-state offset (Vout*-Vout) and the change in the
proportional gain.
[0050] Next, a description is made on that the determination
whether the operation is single or parallel can be made similarly
for a case of the proportional gains of the voltage adjusters 121
of all power conversion units being different from each other. Eqs.
(23), (24), (25), which respectively correspond to Eqs. (1), (2),
(3), are relational equations for the case of the proportional
gains of the voltage adjusters 121 of the power conversion units
being different from each other.
Vout Vout * = i = 1 N Ki i = 1 N Coi s + i = 1 N Ki i = 1 N Coi =
.omega. KN s + .omega. KN , ( 23 ) Vout - Iload = 1 i = 1 N Coi s +
i = 1 N Ki i = 1 N Coi = 1 i = 1 N Ki .omega. KN s + .omega. KN ,
and ( 24 ) Vout = .omega. KN s + .omega. KN ( Vout * - 1 i = 1 N Ki
Iload ) , ( 25 ) ##EQU00014##
where the constant .omega..sub.KN is substituted for terms of the
0-th order of the function s in the denominator and the numerator
on the right hand sides of because it is common to the denominator
and the numerator.
[0051] As with the previous description, the description is made
using FIG. 6 schematically illustrating the single operation of the
one power conversion unit and FIG. 8 schematically illustrating the
parallel operation of the three power conversion units. A relation
corresponding to the Eq. (6) for the single operation and a
relation corresponding to the Eq. (6) for the three-unit parallel
operation are expressed as Eqs. (26) and (27):
Vout = .omega. KN s + .omega. KN ( Vout * - 1 K 1 Iload ) and ( 26
) Vout = .omega. KN s + .omega. KN ( Vout * - 1 K 1 + K 2 + K 3
Iload ) . ( 27 ) ##EQU00015##
[0052] When the proportional gain of the voltage adjuster 121 of
one of the power conversion units is reduced by .DELTA.K, Eqs.
(26), (27) are expressed as Eqs. (28), (29), respectively:
Vout = .omega. KN s + .omega. KN ( Vout * - 1 K 1 - .DELTA. K Iload
) and ( 28 ) Vout = .omega. KN s + .omega. KN ( Vout * - 1 K 1 -
.DELTA. K + K 2 + K 3 Iload ) . ( 29 ) ##EQU00016##
[0053] For the case of the constant resistive load in the steady
state, resistance estimating equations corresponding Eqs. (26),
(28) for the single operation are expressed as Eqs. (30), (31),
respectively:
Rx = Vout K 1 ( Vout * - Vout ) = Vout Iload and ( 30 ) Rx = Vout (
K 1 - .DELTA. K ) ( Vout * - Vout ) = Vout Iload . ( 31 )
##EQU00017##
[0054] Further, the power estimating equations for the constant
power load are expressed as Eqs. (32), (33):
Px=VoutK.sub.1(Vout*-Vout)=VoutIload (32) and
Px=Vout(K.sub.1-.DELTA.K)(Vout*-Vout)=VoutIload (33).
Thus, it is found that in the single operation, the constant
resistive load or the constant power load takes the same estimated
value before and after the proportional gain is changed by
.DELTA.K.
[0055] In contrast to this, constant resistive load estimating
equations corresponding to Eqs. (27), (29) for the three-unit
parallel operation are expressed as Eqs. (34), (35),
respectively:
Rx = Vout K 1 ( Vout * - Vout ) = K 1 + K 2 + K 3 K 1 Vout Iload
and ( 34 ) Rx = Vout ( K 1 - .DELTA. K ) ( Vout * - Vout ) = K 1 -
.DELTA. K + K 2 + K 3 K 1 - .DELTA. K Vout Iload . ( 35 )
##EQU00018##
[0056] Further, constant power load estimating equations are
expressed as Eqs. (36), (37):
Px = Vout K 1 ( Vout * - Vout ) = Vout K 1 K 1 + K 2 + K 3 Iload ,
and ( 36 ) Px = Vout ( K 1 - .DELTA. K ) ( Vout * - Vout ) = Vout K
1 - .DELTA. K K 1 - .DELTA. K + K 2 + K 3 Iload . ( 37 )
##EQU00019##
[0057] As shown above, also in the case of the proportional gains
of the voltage adjusters 121 of all power conversion units being
different from each other, it is found that in the three-unit
parallel operation, the constant resistive load or the constant
power load takes different estimated values before and after the
proportional gain of the voltage adjuster 121 of the one power
conversion unit is changed by .DELTA.K, as with the case of the
common proportional gain.
[0058] From the above description, it is found that determination
whether the operation is single or parallel can be made by changing
the proportional gain to calculate the estimated resistance values
or the estimated power values of the load before and after the
proportional gain is changed and by comparing the change in the
estimated value of the load with the change in the proportional
gain. Since comparing the change in the estimated value of the load
with the change in the proportional gain substitutes, as described
above, for comparison between the change in the steady-state offset
and the change in the proportional gain, the comparison between the
change in the steady-state offset and the change in the
proportional gain is equivalent to the determination of whether the
operation is single or parallel.
[0059] A method is described that actually enables the
determination of whether the operation is single using the
principle of the above-described method of determining the single
operation. FIG. 9 is a flowchart for implementing the determination
of whether the operation is single and shows an operation of the
operation manager 13. The start timing of the process shown by the
flowchart of FIG. 9 can be set arbitrarily. After the process is
started, the load is estimated (Step ST1) using Eq. (9) or Eq.
(10), i.e., using the DC bus voltage Vout and the target current
value Io1* output from the voltage adjuster 121, and then the
estimated value is stored (Step ST2). In the estimation of the load
(Step ST1), the estimation may be made in accordance with the
characteristic of the load: when the load can be simulated as a
constant resistive load, the estimated resistance value Rx of the
constant resistive load is calculated or when the load can be
simulated as a constant resistive load, the estimated power value
Px of the constant power load is calculated, or when the load can
be simulated as either the constant resistive load and the constant
power load, at least one of either values is calculated. After
that, the proportional gain of the voltage adjuster 121 is changed
(Step ST3), to estimate the load (Step ST4). After the proportional
gain is changed, the difference is taken between the last estimated
value and the current estimated value (Step ST5). If the difference
is smaller than a preset value ("Yes" in Step ST5), the count is
incremented (Step ST6) or if the difference is equal to or larger
than the preset value, the count is reset (Step ST7). In the case
of incrementing the count, if the count reaches a number of
determinations ("Yes" in Step ST8), the operation is determined to
be single (Step ST9). When the operation is determined to be
single, the process ends. When the count is reset (Step ST7) or
when the count not reach the number of determinations ("No" in Step
ST8) and the number of gain changes does not reach a preset upper
limit ("No" in Step ST10), the process returns to Step ST2 to store
the estimated value. Then, the process is executed from Step ST3 of
changing the gain. In this way, the gain change is repeated until
the operation is determined to be single (Step ST9) or to be
parallel (Step ST11) when the number of gain changes reaches an
upper limit ("Yes" in Step ST10). Note that the gain is changed
here in such a manner that it is reduced by .DELTA.K from the
initial gain setting Kin the first round, i.e., the gain is set at
(K-.DELTA.K). In the second round, the gain having been
(K-.DELTA.K) is returned to K. In the third round, the gain is set
at (K-.DELTA.K) again. In this way, the gain change is repeated
with the gain being changed alternately to K and (K-.DELTA.K).
[0060] The reason why the operation is determined to be single when
the absolute value of the difference between the current estimated
value and the last estimated value is small and when the count
incremented by multiple changes of the gain reaches the number of
determinations, is for preventing an error in the determination
that the operation is single, due to a transient variation or an
overlap of the process timing with that timing of the other power
conversion units.
[0061] The process may be started at an arbitrary timing, but is
preferably executed at fixed intervals. For example, unit numbers
are assigned to the respective power conversion units and the
process is started at periodic timings in order of the unit numbers
so that the processing periods of the power conversion units are
different from each other, whereby overlap between the process
timings of the power conversion units can be prevented.
[0062] As described above, each power conversion unit of the power
conversion device according to Embodiment 1 is able to determine
that it is in the single operation by comparing change in the
steady-state offset with change in the gain change when changing
its gain alone without obtaining information from the other power
conversion units during the single operation, i.e., the other power
conversion units are not connected in parallel or the other
parallel-connected power conversion units pause their power
conversion operations.
[0063] Note that in the plurality of power conversion units having
the output ports connected in parallel, the operation manager 13
for determining whether the operation is single or parallel may not
be provided to all power conversion units but may be provided to at
least one of the power conversion units.
Embodiment 2
[0064] Embodiment 2 proposes an improvement of efficiency of the
power conversion device for the case of the parallel operation of
the plurality of power conversion units using the single operation
determining method described in Embodiment 1. The whole system of a
power conversion device according to Embodiment 2 has the same
configuration as those shown in FIGS. 1 to 4. Note that the
operation manager 13 according to Embodiment 2 includes an
operation added for the efficiency improvement, compared to the
operation of the operation manager 13 according to Embodiment
1.
[0065] In the following, the operation of each power conversion
unit according to Embodiment 2 is described with reference to FIG.
10 showing the exemplary flowchart of the efficiency improvable
operation of the operation manager 13 of the host power conversion
unit provided with the voltage adjuster 121 shown in FIG. 5 and
FIGS. 11 and 12 for explaining the operation of the host power
conversion unit.
[0066] Using the principle of the single operation determining
method described in Embodiment 1, a method of improving overall
efficiency of the power conversion device is described with
reference to the flowchart shown in FIG. 10.
[0067] The start timing of the process shown by the flowchart of
FIG. 10 can be set arbitrarily. After the process is started, the
load is estimated (Step ST21) using Eq. (9) or Eq. (10). If the
estimated power value is smaller than a preset power threshold
("Yes" in Step ST22), the estimated value is stored (Step ST23) and
then the proportional gain of the voltage adjuster 121 is changed
(Step ST24). If the estimated power value is equal to or exceeds
the preset power threshold ("No" in Step ST22), the process ends.
The power threshold here can be set for each power conversion unit;
for example, the power threshold is set for the efficiency not to
fall below a given value. After changing the proportional gain of
the voltage adjuster 121, the load is estimated (Step ST 25) and
then difference is taken between the last estimated value and the
current estimated value. If the difference is larger than the
preset value ("Yes" in step ST26), the count is incremented (Step
ST27); and if the difference is small and equal to or smaller than
the preset value ("No" in Step ST26), the count is reset (Step
ST28). In the case of incrementing the count, if the count reaches
a number of determinations ("Yes" in Step ST29), the operation is
determined to be parallel (Step ST30) and the power conversion
operation of the relevant power conversion unit is paused (Step
ST31). When the count is reset (Step ST28) or when the count does
not reach the number of determinations ("No" in Step ST29) and the
number of gain changes does not reach the preset upper limit ("No"
in Step ST32), the process returns to Step ST23 to store the
estimated value. The gain change is repeated until the operation is
determined to be parallel and the power conversion operation is
paused (Step ST31) or until the number of gain changes reaches the
upper limit. If the number of gain changes reaches the upper limit
("Yes" in Step ST32) while the count does not reach the number of
determinations, the operation is determined to be single (Step
ST33). In the above process, the gain change is repeated with the
gain being changed alternately to K and (K-.DELTA.K) as described
in Embodiment 1. In addition, the determination in Embodiment 2
that the operation is parallel bears the logic inversion
relationship to the determination in Embodiment 1 that the
operation is single. In further addition, the pause of the power
conversion operation means that the relevant power conversion unit
is in the zero power output state including zero proportional
gain.
[0068] In the above flow, the load estimation in Step ST 21 just
after the start of process requires at least the power estimation
of the constant power load. However, the load estimation in the
later Step ST 25 only requires at least either one of the load
estimation of the constant resistive load or that of the constant
power load depending on the characteristic of the load. It is noted
that when the load estimation is made only for the constant
resistive load in Step ST25, the load estimation is necessarily
made also for the constant resistive load in Step ST21.
[0069] In the flowchart of FIG. 10, the reason why the operation is
determined to be parallel (Step ST30) when the absolute value of
difference between the current estimated value and the last
estimated value is large (Step ST26) and when the count incremented
by multiple changes of the gain reaches the number of
determinations ("Yes" in Step ST29), is for preventing an error in
the determination that the operation is parallel, due to a
transient variation or an overlap of the process timing with that
timing of the other power conversion units.
[0070] A situation is conceivable in which power consumption of the
load increases after the power conversion operation of one of the
power conversion units is paused and only power from the other
power conversion units may become insufficient to supply. This
situation entails drop of the DC bus voltage. Hence, when the DC
bus voltage drops below a preset reference voltage Vout*1 lower
than the target voltage Vout*, the power conversion operation of
the relevant power conversion unit having paused in operation is
resumed and restored again to the operating state.
[0071] When the gain is decreased at the time of changing the gain
in Step ST24 of the process routine shown in FIG. 10, the DC bus
voltage may in some cases drops if the power to the load is
insufficient to supply. In this case, the process is aborted and
the proportional gain is changed to a larger one, for example,
returned to the original value, whereby the DC bus voltage can be
restored to a level equal to or higher than the reference voltage
Vout*1.
[0072] The process may be started at an arbitrary timing, but is
preferably executed at fixed intervals. For example, unit numbers
are assigned to the respective power conversion units and the
process is started at periodic timings in order of the unit numbers
so that the processing periods of the power conversion units are
different from each other, whereby overlap between successive
process timings of the power conversion units can be prevented.
[0073] Next, a specific example of improving the efficiency is
described with reference to the process flowchart of FIG. 10. FIG.
12 is an example operation for improving the efficiency of a power
conversion device system that is configured with two of the power
conversion units: a power conversion unit A and a power conversion
unit B having efficiency characteristics shown in FIG. 11. It is
assumed here that the voltage adjusters 121 of the power conversion
units A and B have a common proportional gain and the proportional
gain is changed alternately to values of 1.0 time and 0.5 times of
the design value.
[0074] As shown in FIG. 11, in the case of the power conversion
units A and B having the common proportional gain, the respective
operating points of the power conversion units are as indicated by
the black circles and the respective output powers of the power
conversion units are balanced at a total output power of 1,100 W.
At that time, the total input to the power conversion units A and B
is 1,238 W and the overall efficiency of the power conversion
device is 88.9%. Since the output power of the power conversion
unit B is smaller than the power threshold, the process shown in
the flowchart of FIG. 10 is started.
[0075] As shown in the upper graph of FIG. 12, changing 0.5 times
the proportional gain of the power conversion unit B increases the
output power of the power conversion unit A and decreases the
output power of the power conversion unit B. Based on the power
change caused by the proportional gain change, the operation is
determined to be parallel as described above, and the operation of
the power conversion unit B is paused and only power conversion
unit A is operated as shown in the lower graph of FIG. 12. At that
time, the operating point of the power conversion unit A is shifted
to the point of 1,100 W indicated by the black circle and the input
becomes 1,183 W; accordingly, the efficiency is 93.0%. In this way,
the overall efficiency of the power conversion device can be
improved from 88.9% before the pause of power conversion unit B to
93.0% after the pause.
[0076] As described above, it is possible according to Embodiment 2
to improve the overall efficiency of the power conversion units as
a whole by determining whether or not the other power conversion
units are in the power conversion operations when one of the power
conversion unit operates at or below its power threshold and by
pausing the operation of the one power conversion unit if the other
power conversion units are in the power conversion operations.
Embodiment 3
[0077] Embodiment 3 describes an efficiency improvement for a load
including a power generation mechanism, such as a solar
photovoltaic power generation, a wind power generation, or a hydro
power generation, using the efficiency improvement method described
in Embodiment 2, for the power conversion device operating the
plurality of power conversion units parallelly.
[0078] FIG. 13 is a block diagram showing an example of a power
conversion system including the power conversion device, according
to Embodiment 3. Embodiment 3 and Embodiment 2 are different in
that the load 30 includes a power generation mechanism 32, such as
a solar photovoltaic power generation, a wind power generation, or
a hydro power generation, in addition to a power consuming load 31.
The power conversion units in the power conversion device according
to Embodiment 3 are similar in configuration and operation to the
power conversion device shown in FIGS. 1 to 12. In the following,
the configuration and the operation of the power conversion units
according to Embodiment 3 are described with reference to FIGS. 1
to 12 and FIG. 13 in which the load 30 shown in FIGS. 1, 2 is
replace with that corresponding to Embodiment 3. Note that in the
description of the operation of the power conversion unit, a
description overlapped with that in Embodiment 2 is omitted and an
operation associated with the difference in the configuration of
the load 30 is described.
[0079] The load 30 shown in FIG. 13 configured with a power
consuming load 31 and the power generation mechanism 32. The load
current Iload takes a positive value when the power consumed by the
power consuming load 31 is larger than the generated power of the
power generation mechanism 32 and takes a negative value when the
power consumed by the power consuming load 31 is smaller than the
generated power of the power generation mechanism 32. When the load
current is positive, the power converter 11 operates to convert the
power from the power source to that to the load. When the load
current is negative, on the other hand, the power converter
operates 11 to convert the power from the load to that to the power
source. In other words, the power converter 11 connected between
the power source and the load is able to perform power conversion
from the power source to the load and also from the load to the
power source, i.e., operates as a bidirectional power converter.
The negative state of the load current Iload means that the actual
voltage Vout is higher than the target voltage Vout* because a
power is supplied to the power conversion units from the load 30;
hence, the estimated resistance value Rx and the estimated power
value Px expressed by Eqs. (9), (10) take negative values. When a
power is supplied to the power conversion units from the load 30,
the power conversion units to which the power is supplied returns
the power to the power source, for example, to charge a storage
battery connected to the power source. The efficiency improvement
method described in Embodiment 2 is likewise applicable to the
configuration of Embodiment 3 because the operation is determined
to be parallel on the basis of the absolute value of the difference
between the estimated values Rx or between the estimated values Px
before and after the proportional gain is changed (Step ST26 in
FIG. 10).
[0080] In the negative state of the load current Iload, a situation
is conceivable in which the generated power of the generation
mechanism 32 in the load increases after one of the power
conversion units pauses in operation and the generated power
becomes too much for only power conversion units other than that
paused in operation to charge. This situation entails further
increase of the DC bus voltage. Hence, when the DC bus voltage
exceeds a preset reference voltage Vout*2 higher than the target
voltage Vout*, the power conversion operation of the relevant power
conversion unit having paused in operation is resumed and restored
again to the operating state. In addition, since rise of the DC bus
voltage above the target voltage Vout* occurs only in the case of
including the power generation mechanism in the load, there is no
need to monitor the direction of the load current. The only need is
to monitor the DC bus voltage.
[0081] When the gain is decreased at the time of changing the gain
in Step ST24 of the process routine shown in FIG. 10, the DC bus
voltage may in some cases exceed the reference voltage Vout*2 if
the generated power of the load is too much to charge. In this
case, the process is aborted and the proportional gain is changed
to a larger one, for example, returned to the original value,
whereby the DC bus voltage can be restored to a level equal to or
lower than the reference voltage Vout*2. In other words, by setting
lower than the target voltage of the voltage controller 12 the
reference voltage Vout*1, which is described in Embodiment 2, for
releasing a pause of the operation at an overload and by setting
higher than the target voltage of the voltage controller 12 the
reference voltage Vout*2, which is described above, for releasing a
pause of the operation at an overgeneration, the DC bus voltage can
be always kept between the reference voltage Vout*1 and the
reference voltage Vout*2, thus being able to implement a stable
operation capable of the bidirectional power transfer.
[0082] It should be noted that each embodiment may be combined or
appropriately modified or omitted.
REFERENCE NUMERALS
[0083] 10, 10-1, 10-2, . . . , 10-N: power conversion unit; [0084]
11: power converter; [0085] 12: voltage controller; [0086] 13:
operation manager; [0087] 15: voltage sensor; [0088] 20-1, 20-2, .
. . , 20-N: DC power source; [0089] 21-1, 21-2, . . . , 21-N: AC
power source; [0090] 30: load; [0091] 31: power consuming load;
[0092] 32: power generation mechanism; and [0093] 121: voltage
adjuster.
* * * * *